Hafnium oxide HfO2

spore: profile
id: hafnium-oxide
spore_version: 0.2
family: oxide-ceramics
name: Hafnium oxide
abbreviation: HfO2
role: high-k gate dielectric and thermal barrier coating oxide
category: ceramics
type: oxides
summary: |
  Hafnium oxide (hafnia, HfO2) is a white to off-white, dense, very high-melting oxide. Supplier safety data sheets give its density as 9.68 g/cc and its melting point as 2774 °C and 2758 °C. A database record gives 2800 °C, and a Toshiba paper quotes 2900 °C for hafnia. Its main industrial use is as a high-k dielectric in transistors, where a review table gives a dielectric constant of 22 to 25 for HfO2 against about 3.9 for silicon dioxide. Hafnia-based coatings are also studied for gas-turbine thermal barriers, where a 2004 Toshiba paper reports better sintering resistance than yttria-stabilized zirconia. The space record is ground simulation only: a 2024 study measured how simulated near-Earth conditions lowered the laser damage threshold of HfO2/SiO2 mirror films.
best_for: High-k dielectric films for transistors (dielectric constant 22 to 25 in a review table) and high-melting thermal barrier coatings for hot parts
weakest_at: Bulk mechanical data were not found in a source opened for this profile, and the space record is a ground simulation that cut the laser damage threshold by up to 46.80% for debris impact
identity:
  formula: HfO2
  cas: 12055-23-1
  crystal_system: monoclinic at low temperature (tetragonal and cubic forms at higher temperature)
  source: kol-hfo2-review
  where: Section 3 (Hf-based high-k dielectrics), first paragraph; CAS 12055-23-1 printed in Section 3 of the Praxair SDS; formula HfO2 in the PubChem record (CID 292779)
tags:
  environments: []
  stresses:
    - atomic-oxygen
    - extreme-heat
  applications:
    - optics-and-cameras
    - electronics-and-chips
    - propulsion-and-tanks
properties:
  - benchmark: density
    label: Density, Angstrom Sciences SDS
    value: 9.68
    unit: g/cc
    scope: material
    conditions:
      form: product as sold (SDS Section 9 lists powder, granules, pellets, sputtering target and custom parts)
      temperature: not stated on the page
    method: Value printed in a supplier safety data sheet
    status: ground
    source: angstrom-sds
    where: Section 9, Density row
  - benchmark: melting_point
    label: Melting point, Angstrom Sciences SDS
    value: 2774
    unit: °C
    scope: material
    conditions:
      form: hafnium oxide, product as sold
      pressure: not stated on the page
    method: Value printed in a supplier safety data sheet
    status: ground
    source: angstrom-sds
    where: Section 9, Melting Point row
  - benchmark: melting_point
    label: Melting point at 1 atm, Praxair SDS
    value: 2758
    unit: °C
    scope: material
    conditions:
      pressure: 1 atm
      form: hafnium (IV) oxide, Praxair product P-6348
    method: Value printed in a supplier safety data sheet (printed as 4996°F (2758°C))
    status: ground
    source: praxair-sds
    where: Section 9, MELTING POINT at 1 atm row
  - benchmark: melting_point
    label: Melting point, PubChem record
    value: 2800
    unit: °C
    scope: material
    conditions:
      basis: experimental value in the PubChem record (grade and method not stated)
    method: Experimental value quoted in a chemical database
    status: ground
    source: pubchem-hfo2
    where: Chemical and Physical Properties, Experimental Properties, Melting Point (reference 21)
  - benchmark: melting_point
    label: Melting point of hafnia, Toshiba paper
    value: 2900
    unit: °C
    scope: material
    conditions:
      basis: hafnia (HfO2) as quoted in the introduction, not a measured value from the paper
    method: Value stated in a conference-style paper introduction
    status: ground
    source: toshiba-hafnia-tbc
    where: Section 1 (Introduction), second paragraph
  - benchmark: boiling_point
    label: Boiling point at 1 atm, Praxair SDS
    value: 5400
    unit: °C
    scope: material
    conditions:
      pressure: 1 atm
      form: hafnium (IV) oxide, Praxair product P-6348
    method: Value printed in a supplier safety data sheet (printed as 9752°F (5400°C))
    status: ground
    source: praxair-sds
    where: Section 9, BOILING POINT at 1 atm row
  - benchmark: boiling_point
    label: Boiling point, PubChem record
    value: 5400
    unit: °C
    scope: material
    conditions:
      pressure: 760 mm Hg
    method: Experimental value quoted in a chemical database
    status: ground
    source: pubchem-hfo2
    where: Chemical and Physical Properties, Experimental Properties, Boiling Point (reference 21)
  - benchmark: dielectric_constant
    label: Dielectric constant range, HfO2 (high-k comparison)
    value: 22
    value_high: 25
    bound: range
    unit: '-'
    scope: material
    conditions:
      basis: range printed for HfO2 in a review table comparing high-k oxides
      form: film or bulk not stated in the table
    method: Range compiled in a review table, which cites refs 24 to 28
    status: ground
    source: kol-hfo2-review
    where: Table I, HfO2 row (dielectric constant column)
  - benchmark: band_gap
    label: Band gap, monoclinic HfO2
    value: 5.65
    unit: eV
    scope: material
    conditions:
      phase: monoclinic (m-HfO2)
    method: Value quoted in review text, which cites refs 30 and 45 for the phase dependence
    status: ground
    note: The review says the band gap changes with crystal phase. The value is not in property-canon, so it uses a short snake_case name.
    source: kol-hfo2-review
    where: Section 3 (Hf-based high-k dielectrics), paragraph on band gap by phase
  - benchmark: band_gap
    label: Band gap, tetragonal HfO2
    value: 5.79
    unit: eV
    scope: material
    conditions:
      phase: tetragonal (t-HfO2)
    method: Value quoted in review text, which cites refs 30 and 45 for the phase dependence
    status: ground
    source: kol-hfo2-review
    where: Section 3 (Hf-based high-k dielectrics), paragraph on band gap by phase
  - benchmark: band_gap
    label: Band gap, cubic HfO2
    value: 5.53
    unit: eV
    scope: material
    conditions:
      phase: cubic (c-HfO2)
    method: Value quoted in review text, which cites refs 30 and 45 for the phase dependence
    status: ground
    source: kol-hfo2-review
    where: Section 3 (Hf-based high-k dielectrics), paragraph on band gap by phase
  - benchmark: band_gap
    label: Band gap, amorphous HfO2
    value: 6.04
    unit: eV
    scope: material
    conditions:
      phase: amorphous (a-HfO2)
    method: Value quoted in review text, which cites refs 30 and 45 for the phase dependence
    status: ground
    source: kol-hfo2-review
    where: Section 3 (Hf-based high-k dielectrics), paragraph on band gap by phase
  - benchmark: thermal_conductivity
    label: Thermal conductivity of hafnia, Toshiba paper
    value: 1.5
    unit: W/mK
    scope: material
    conditions:
      basis: hafnia (HfO2) as quoted in the introduction and Figure 1 of the paper; temperature and grain size not stated
    method: Value stated in a paper introduction (printed as 1.5 W/mK)
    status: ground
    note: Stated in the same sentence as the 2900 °C melting point. The paper's own coating is yttria-stabilized hafnia, not pure HfO2, so this is the figure for hafnia only.
    source: toshiba-hafnia-tbc
    where: Section 1 (Introduction), second paragraph
  - benchmark: bulk_density_powder
    label: Bulk density of powder, Praxair SDS
    value: 1922
    value_high: 2883
    bound: range
    unit: kg/m3
    scope: material
    conditions:
      form: powder, as printed in lb/ft3 and converted in parentheses in the SDS
    method: Range printed in a supplier safety data sheet (printed as 120-180 lb/ft3 (1922-2883 kg/m3))
    status: ground
    source: praxair-sds
    where: Section 9, BULK DENSITY row
corrosion:
  - environment: Simulated near-Earth space (proton irradiation, atomic oxygen and debris impact) on HfO2/SiO2 mirror films
    headline: Simulated space conditions lowered the laser damage threshold of HfO2/SiO2 films, most after debris impact.
    effect: The study reports damage-threshold losses of 15.38% after proton irradiation, 13.12% after atomic oxygen and 46.80% after simulated debris penetration, and 63.19% with all three combined.
    rating: moderate
    conditions:
      film: HfO2/SiO2 triple-band high-reflection film on fused silica
      basis: ground simulation, not an in-orbit test
    source: hplse-2024
    where: Abstract; the article body describes the tests as ground simulation
  - environment: High temperature in air (hafnia-based thermal barrier coating)
    headline: A hafnia-based coating resists sintering better than yttria-stabilized zirconia by about 100 °C.
    effect: The paper says sintering of YSZ coatings occurs above a surface temperature of about 1200 °C, and that the sintering resistance of its 7.5 mass% yttria-hafnia coating was improved by about 100 °C over 8 mass% yttria-zirconia.
    rating: low
    conditions:
      coating: 7.5 mass% Y2O3 in HfO2 (7.5YSH), EB-PVD top coat about 200 µm, not pure HfO2
      basis: thermal exposure test; the paper reports sintering behaviour at 1300 °C and 1400 °C
    source: toshiba-hafnia-tbc
    where: Abstract; Section 1 (Introduction); sintering results section
  - environment: Heating (crystal phase change)
    headline: Hafnia changes crystal form on heating, from monoclinic to tetragonal and then cubic.
    effect: The review says HfO2 is monoclinic at low temperature, changes to tetragonal on heating and to cubic at higher temperature, and that the change in crystal structure changes the band gap.
    rating: low
    conditions:
      form: pure HfO2
      temperature: transition temperatures not given in the review
    source: kol-hfo2-review
    where: Section 3 (Hf-based high-k dielectrics), first paragraph
  - environment: Strong oxidizing agents
    headline: The SDS lists strong oxidizing agents as incompatible with hafnium oxide.
    effect: The Praxair SDS lists strong oxidizing agents as incompatible materials, and says that heating to decomposition can give toxic fumes and gases.
    rating: low
    conditions:
      form: powder
      basis: incompatibility as listed in the SDS, not a corrosion rate
    source: praxair-sds
    where: Section 10 (Stability and Reactivity), Incompatible materials row
risks:
  hazards:
    - text: The Praxair SDS (2006) carries the headline CAUTION! Irritating powder. It says the product may cause eye and skin irritation, and respiratory and digestive tract irritation.
      source: praxair-sds
      where: Section 2 (Hazards Identification), Emergency overview
    - text: The Angstrom Sciences SDS (revised 07/10/15) gives no signal word and no hazard statements, and an HMIS health rating of 1.
      source: angstrom-sds
      where: Section 2 (Hazard Identification)
    - text: 'PubChem reports that hafnium dioxide is not classified under GHS: 337 of 344 company reports say it does not meet GHS hazard criteria.'
      source: pubchem-ghs
      where: Safety and Hazards, GHS Classification (CID 292779)
  failure_risks:
    - mode: Sintering of a hafnia-based thermal barrier coating
      trigger: Long thermal exposure at high temperature (the paper tests 1300 °C and 1400 °C)
      consequence: The paper reports that sintering of its 7.5YSH coating started at 1300 °C. Its sintering resistance is about 100 °C better than an 8YSZ coating, but it still sinters at high temperature.
      severity: moderate
      source: toshiba-hafnia-tbc
      where: Abstract; sintering results section
    - mode: Loss of laser damage threshold in space-like conditions
      trigger: Proton irradiation, atomic oxygen or debris impact on HfO2/SiO2 mirror films (ground simulation)
      consequence: The damage threshold falls by 15.38%, 13.12% and 46.80% for the single factors and by 63.19% with all three combined, in a ground simulation.
      severity: moderate
      source: hplse-2024
      where: Abstract
    - mode: Phase change on heating
      trigger: Heating to high temperature
      consequence: The crystal structure changes from monoclinic to tetragonal and then cubic, and the band gap changes with it, so optical and electrical properties can shift with temperature.
      severity: low
      source: kol-hfo2-review
      where: Section 3 (Hf-based high-k dielectrics)
  side_effects: []
  handling:
    - text: Use only with adequate ventilation or respiratory protection, avoid inhaling the powder, and avoid contact with eyes, skin and clothing.
      source: praxair-sds
      where: Section 7 (Handling and Storage), Precautions to be taken in handling
    - text: Store in a cool, dry place in a tightly sealed container, and do not blow dust off clothing or skin with compressed air.
      source: angstrom-sds
      where: Section 7 (Handling and Storage)
    - text: The exposure limit is 0.5 mg/m3 as Hf (OSHA PEL and ACGIH TLV-TWA, long-term value).
      source: praxair-sds
      where: Section 8 (Exposure Controls), Hafnium Oxide row and footnotes
  mitigations:
    - text: The Praxair SDS says local exhaust is insufficient, and that mechanical ventilation is required.
      source: praxair-sds
      where: Section 8 (Engineering Controls)
    - text: Vacuum up spilled powder with a HEPA filter and place it in a closed container for disposal.
      source: angstrom-sds
      where: Section 6 (Accidental Release Measures)
uses:
  - application: electronics-and-chips
    device: High-k gate dielectric in CMOS transistors
    example: The review says that Intel's 45 nm and 32 nm high-k metal-gate transistors replaced the silicon oxide insulator with hafnium-based oxides.
    source: kol-hfo2-review
    where: Section 1 (Introduction) and Section 2 (Gate dielectrics)
  - application: propulsion-and-tanks
    device: Hafnia-based thermal barrier coating on gas-turbine blades (EB-PVD top coat)
    example: The Toshiba paper develops a 7.5 mass% yttria-hafnia top coat for gas-turbine blades, and reports better sintering resistance than an 8 mass% yttria-zirconia coating.
    source: toshiba-hafnia-tbc
    where: Abstract; Section 1 (Introduction)
  - application: optics-and-cameras
    device: HfO2/SiO2 triple-band high-reflection optical film (ground-tested for space conditions)
    example: A 2024 study measured how simulated near-Earth space conditions reduced the laser damage threshold of these films, for proton irradiation, atomic oxygen and debris impact.
    source: hplse-2024
    where: Abstract; title of the article
supply:
  - subject: Hafnium (metal, as reported by USGS)
    abundance: Crustal abundance was not found in a source opened for this profile
    producers: []
    critical_lists:
      - list: USGS 2025 List of Critical Minerals (hafnium is on the final list)
        listed: true
    recycling: 'USGS: hafnium metal recycling was minimal.'
    substitutes:
      - 'USGS: boron or cadmium-silver-indium alloys can substitute for hafnium metal in nuclear control rods.'
    source: usgs-mcs2026-zirhf
    where: 'Zirconium and Hafnium chapter: Domestic Production and Use (one producer in Oregon and one in Utah); World Mine Production and Reserves (world primary hafnium production data not available); Import Sources 2021 to 2024 for unwrought hafnium (Germany 54%, China 21%, France 12%, United Kingdom 8%, other 5%; these are US import shares, not production shares); Recycling; Substitutes. The critical-list entry is confirmed on the USGS 2025 list page (usgs-crit2025).'
documents:
  - title: 'PubChem: Hafnium dioxide (CID 292779), experimental properties and GHS classification'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/292779/JSON
    year: 2026
  - title: 'Praxair Material Safety Data Sheet: Hafnium Oxide, MSDS No. P-6348 (February 2006), copy hosted by UCSB'
    type: sds
    url: https://wiki.nanofab.ucsb.edu/w/images/2/2d/Hafnium_Oxide_MSDS.pdf
    year: 2006
  - title: 'Angstrom Sciences: Hafnium Oxide safety data sheet (revised 07/10/15)'
    type: sds
    url: https://angstromsciences.com/wp-content/uploads/files/SDS/Hafnium%20Oxide.pdf
    year: 2015
  - title: 'USGS Mineral Commodity Summaries 2026: Zirconium and Hafnium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zirconium-hafnium.pdf
    year: 2026
  - title: 'Kol and Oral: Hf-Based High-k Dielectrics: A Review (Acta Physica Polonica A, open access)'
    type: paper
    url: https://przyrbwn.icm.edu.pl/APP/PDF/136/app136z6p05.pdf
    year: 2019
  - title: 'Matsumoto et al.: Microstructure and Sintering Behavior of Hafnia-based Thermal Barrier Coating by EB-PVD Process (Trans. MRS-J 29)'
    type: paper
    url: https://www.mrs-j.org/pub/tmrsj/vol29_no2/vol29_no2_377.pdf
    year: 2004
  - title: Influence of space environmental factors on the laser-induced damage thresholds in optical components (High Power Laser Science and Engineering)
    type: paper
    url: https://www.cambridge.org/core/journals/high-power-laser-science-and-engineering/article/influence-of-space-environmental-factors-on-the-laserinduced-damage-thresholds-in-optical-components/915899B3DF413B4BEB33E12DAE4F36B2
    year: 2024
  - title: 'NIST: Atomic Weights and Isotopic Compositions for All Elements (hafnium)'
    type: database
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Hf&all=all&ascii=html
    year: 2026
gaps:
  - No bulk mechanical data (elastic modulus, flexural strength, hardness, fracture toughness) for HfO2 was found in a source opened for this profile. A Springer review of hafnia-toughened ceramics was seen in search results only and not read.
  - No coefficient of thermal expansion from a manufacturer or primary source was found. A generic database value seen in search results was not used.
  - No measured thermal conductivity of bulk monoclinic HfO2 was found in an open source. The arXiv preprint 2003.07546 was read but is about hafnon (HfSiO4), so its 13.3 W/mK is not used. The 1.5 W/mK figure is an introduction value from the Toshiba paper with no temperature given.
  - Density from PubChem (9.68 @25 °C) is left out because the record prints no unit.
  - The melting point values differ (2758, 2774, 2800 and 2900 °C). All are kept with their sources. The supplier SAM sputtering-target page gives 2900 °C and was seen in search results only.
  - The band gap is not in property-canon, so benchmark band_gap is a new snake_case name.
  - Oxygen's atomic weight is not given as one value. NIST prints an interval (15.99903 to 15.99977), so no atomic_weight is given for oxygen.
  - Crustal abundance of hafnium was not found in a source opened for this profile.
  - World primary hafnium production data are not available (USGS). The producers list is empty, and the import shares for unwrought hafnium are recorded in the supply where field, not as production shares.
  - 'Hazard classification conflicts: the Praxair SDS (2006) gives an irritant warning, the Angstrom SDS (2015) gives none, and PubChem reports Not Classified. A Materion SDS was seen in search results but not read, and no current supplier SDS was read.'
  - The Toshiba sintering and thermal values come from a yttria-stabilized hafnia coating (7.5YSH), not pure HfO2. They are labelled that way in the file.
  - No flight test of HfO2 was found. The space record is a ground simulation (2024 HPLSE paper) and the MISSE or NASA flight records do not cover HfO2 in the sources opened.
  - Water solubility is printed as insoluble in two SDS sheets. It is not a numeric property, so it is not listed as a value.
sources:
  angstrom-sds:
    title: 'Angstrom Sciences: Hafnium Oxide safety data sheet'
    id: Angstrom Sciences SDS, last revision 07/10/15, CAS 12055-23-1
    url: https://angstromsciences.com/wp-content/uploads/files/SDS/Hafnium%20Oxide.pdf
    year: 2015
    kind: datasheet
    license: supplier document, linked not reproduced
  praxair-sds:
    title: 'Praxair Material Safety Data Sheet: Hafnium Oxide (MSDS No. P-6348, February 2006)'
    id: Praxair P-6348, February 2006; copy hosted by UCSB Nanofab wiki
    url: https://wiki.nanofab.ucsb.edu/w/images/2/2d/Hafnium_Oxide_MSDS.pdf
    year: 2006
    kind: datasheet
    license: supplier document, linked not reproduced
  pubchem-hfo2:
    title: 'PubChem compound record: Hafnium dioxide (CID 292779)'
    id: CID 292779; molecular formula HfO2; CAS 12055-23-1
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/292779/JSON?heading=Experimental+Properties
    year: 2026
    kind: database
    license: public domain (NCBI)
  pubchem-ghs:
    title: 'PubChem compound record: Hafnium dioxide (CID 292779), safety and hazards'
    id: CID 292779; GHS classification section
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/292779/JSON
    year: 2026
    kind: database
    license: public domain (NCBI)
  usgs-mcs2026-zirhf:
    title: 'Mineral Commodity Summaries 2026: Zirconium and Hafnium'
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zirconium-hafnium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-crit2025:
    title: About the 2025 List of Critical Minerals (USGS, final list)
    id: Final 2025 List of Critical Minerals, November 2025
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
    kind: web
    license: public domain (USGS)
  kol-hfo2-review:
    title: 'Kol and Oral: Hf-Based High-k Dielectrics: A Review (Acta Physica Polonica A, 136)'
    id: Acta Physica Polonica A 136, open access repository copy
    url: https://przyrbwn.icm.edu.pl/APP/PDF/136/app136z6p05.pdf
    year: 2019
    kind: paper
    license: open access (journal repository)
  toshiba-hafnia-tbc:
    title: 'Matsumoto, Itoh, Ishiwata and Kato: Microstructure and Sintering Behavior of Hafnia-based Thermal Barrier Coating by EB-PVD Process'
    id: Transactions of the Materials Research Society of Japan 29 [2] 377-380 (2004)
    url: https://www.mrs-j.org/pub/tmrsj/vol29_no2/vol29_no2_377.pdf
    year: 2004
    kind: paper
    license: publisher PDF, linked not reproduced
  hplse-2024:
    title: Influence of space environmental factors on the laser-induced damage thresholds in optical components (High Power Laser Science and Engineering, 2024)
    id: High Power Laser Science and Engineering, January 2024 (DOAJ listed)
    url: https://www.cambridge.org/core/journals/high-power-laser-science-and-engineering/article/influence-of-space-environmental-factors-on-the-laserinduced-damage-thresholds-in-optical-components/915899B3DF413B4BEB33E12DAE4F36B2
    year: 2024
    kind: paper
    license: open access journal (DOAJ listed); licence not checked
  nist-atomic-weights:
    title: 'NIST: Atomic Weights and Isotopic Compositions for All Elements'
    id: NIST Physical Measurement Laboratory, hafnium (Z=72) and oxygen (Z=8)
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Hf&all=all&ascii=html
    year: 2026
    kind: database
    license: public domain (NIST)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 14
  unreachable: 0
  caveats: 4
  note: 'Read by an AI assistant on 2026-10-26 from the PubChem record, two SDS copies, the USGS 2026 chapter, the Kol and Oral review, the Toshiba paper, the 2024 HPLSE paper and NIST. The arXiv preprint 2003.07546 was read and not used because it is about hafnon. Caveats: melting points disagree across sources, the Toshiba values are for yttria-stabilized hafnia, the Praxair SDS dates from 2006, and the space record is ground simulation.'
story:
  recipe:
    line: HfO₂ is a metal oxide. Each formula unit combines one hafnium atom with two oxygen atoms. In the oxide the hafnium is Hf⁴⁺, so two O²⁻ ions balance it. The powder is insoluble in water.
    parts:
      - name: Hafnium
        symbol: Hf
        atoms: 1
        atomic_weight: 178.49
        role: The metal. Hafnium sits in the oxide as Hf⁴⁺, the hafnium(IV) oxide form named in the Praxair SDS, and its oxide has a crystal structure similar to zirconia (Toshiba paper).
      - name: Oxygen
        symbol: O
        role: Two oxygen atoms per formula unit balance the charge of the Hf⁴⁺ ion.
  risk_groups:
    - title: For people
      level: 1
      verdict: Irritant powder, not classified in some records
      lead: The supplier SDS sheets and the database disagree. The 2006 Praxair SDS warns of eye and skin irritation, while the 2015 Angstrom SDS and PubChem give no hazard statement.
      points:
        - label: The powder
          line: The Praxair SDS gives the warning CAUTION! Irritating powder, with eye and skin irritation and respiratory and digestive tract irritation. The Angstrom SDS gives no signal word and an HMIS health rating of 1.
        - label: Exposure limit
          line: The OSHA PEL and ACGIH TLV-TWA are 0.5 mg/m³ as Hf (long-term value).
        - label: Classification
          line: PubChem reports Not Classified under GHS, with 337 of 344 company reports saying the product does not meet GHS criteria.
    - title: If it fails
      lead: These results come from supplier tests, a thermal-barrier paper and a ground simulation of space conditions. No service incidents were found in the sources opened.
      points:
        - label: Sintering
          line: The Toshiba paper reports that its hafnia-based coating starts to sinter at 1300 °C, about 100 °C later than an 8YSZ coating.
        - label: Space conditions
          line: In a ground simulation of proton, atomic-oxygen and debris damage, the laser damage threshold of HfO2/SiO2 films fell by 63.19% with all three factors combined.
        - label: Phase change
          line: Heating changes the crystal form from monoclinic to tetragonal and then cubic, and the band gap changes with it.
    - title: Handling and storage
      points:
        - label: Keep it dry and sealed
          line: Store in a cool, dry place in a tightly sealed container. Do not blow dust off clothing or skin with compressed air.
        - label: Avoid dust
          line: Use adequate ventilation or respiratory protection, avoid inhaling the powder, and use a HEPA vacuum for spills.
        - label: Keep it apart
          line: Keep it away from strong oxidizing agents. Heating to decomposition can give toxic fumes and gases.
  why: Hafnium oxide (hafnia, HfO₂) is a dense, very high-melting oxide. It is used as a high-k dielectric in transistors, where it replaced silicon oxide, and it is studied as a thermal barrier coating for gas-turbine blades. Its limits are the lack of bulk mechanical data in the sources opened, conflicting hazard classifications, and a space record that is a ground simulation only.
  uses:
    - where: Transistor gate dielectric
      line: The review says that Intel's 45 nm and 32 nm high-k metal-gate transistors replaced silicon oxide with hafnium-based oxides.
    - where: Gas-turbine thermal barrier coating
      line: A Toshiba paper develops a hafnia-based top coat that resists sintering better than an 8YSZ coating by about 100 °C. Its coating is yttria-stabilized hafnia.
    - where: Optical mirror films for space
      line: A 2024 study tested HfO2/SiO2 high-reflection films under simulated near-Earth space conditions and measured the loss of laser damage threshold.
  can_do:
    - label: High-k dielectric
      value: 22 to 25
      line: A review table gives a dielectric constant of 22 to 25 for HfO2, well above that of silicon dioxide.
    - label: High melting point
      value: 2774 °C
      line: Supplier SDS sheets give 2774 °C and 2758 °C, and PubChem gives 2800 °C. The Toshiba paper quotes 2900 °C for hafnia.
    - label: Wide band gap
      value: 5.53 to 6.04 eV
      line: The review gives band gaps of 5.53 eV (cubic), 5.79 eV (tetragonal), 5.65 eV (monoclinic) and 6.04 eV (amorphous).
    - label: Dense oxide
      value: 9.68 g/cc
      line: The Angstrom SDS gives 9.68 g/cc, and the Praxair SDS gives a specific gravity of 9.68 (water = 1).
  compare: The supplier sheets agree on density, at 9.68. They disagree on melting point, with 2758 °C and 2774 °C from the two SDS sheets, 2800 °C from PubChem and 2900 °C from the Toshiba paper. Thermal conductivity is 1.5 W/mK, given in the Toshiba paper for hafnia without a temperature, so it is the only value for that property here.
  compare_rows:
    - label: Melting point
      benchmark: melting_point
    - label: Heat conduction
      benchmark: thermal_conductivity
    - label: Density
      benchmark: density
  handling:
    - Handle and store the powder in a cool, dry, sealed container. Avoid breathing dust and keep it away from strong oxidizing agents.
    - Use local exhaust or respiratory protection, and keep to the 0.5 mg/m³ limit as Hf.
    - Clean spills with a HEPA vacuum rather than dry sweeping or compressed air.
  good_for:
    - High-k dielectric films for transistors. A review table gives a dielectric constant of 22 to 25 and says HfO2 replaced silicon oxide in Intel's high-k gates.
    - High-melting thermal barrier coatings for hot parts. The Toshiba paper reports better sintering resistance than 8YSZ.
    - Optical mirror films. A 2024 study measured how simulated space conditions reduced the laser damage threshold of HfO2/SiO2 films.
  watch_out:
    - Conflicting hazard data. The 2006 Praxair SDS warns of irritation, while the 2015 Angstrom SDS and PubChem do not. Check the current supplier SDS before handling.
    - Space data are ground simulation only. The 2024 study found a 63.19% loss of laser damage threshold with all three factors, and no in-orbit result was found.
    - Phase change on heating. Hafnia changes from monoclinic to tetragonal and then cubic, which changes its band gap.
  verdict: Hafnium oxide is a strong candidate for high-k dielectric films and for high-melting thermal barrier coatings, and the sources give its melting point as between 2758 and 2900 °C. Bulk mechanical data, a measured thermal conductivity for pure HfO2 and a flight record are missing, and the hazard classification is not settled, so the profile is a draft with those gaps stated.